The present invention relates to a power assisted vehicle steering system and gear and to a power assistance means for a vehicle steering gear.
Power assisted vehicle steering gears include a servo motor which is controlled in response to a steering input assist displacement of a steering output component which effects the steering maneuver. Usually the servo motor is in the form of an hydraulic ram or electric motor. Control means for the servo motor and which is responsive to the torque applied during the steering input is usually in the form of an hydraulic valve (having parts which are relatively displaceable against resilient biasing and in response to the input torque), electric micro-switches or load cells all of which are well known in the art. It is conventional for the servo motor and its control means to be incorporated in a power assisted vehicle steering gear as an integral structural part of that gear. As a consequence of necessary additional components, and complexity of manufacture and assembly, power assisted steering gears are inevitably more expensive than wholly manual steering gears. Furthermore, it is generally recognized that a decision has to be made during the original design and costing of a particular vehicle whether the steering gear on that vehicle is to be manual or power assisted; if this decision is for a manual gear then it is usually considered as impractical and uneconomic to replace the manual gear by a power assisted gear during the life of the vehicle. However, it is apparent that there is a requirement for a relatively inexpensive power assisted steering gear which may easily be fitted as original equipment to a vehicle and may be particularly suitable to achieve as a conversion of an existing wholly manual steering gear. It is an object of the present invention to satisfy the aforementioned requirement (even bearing in mind that one appropriate means for converting a manual steering gear into a power assisted steering gear has already been proposed in our British Pat. Specification No. 2,l97,827A).